This is a key plot point in Goblet of Fire, but I think that it must've gone a long time without being mentioned. The fact that she's been banned gets mentioned again at some point between the second and third tasks, but my reaction was "She's been banned? When did that happen?". So, just to save myself a reread, I'm asking here - When did she actually get banned?
The reason and exact time she got banned are never mentioned. All we get is this line from Ron:
"We'd have seen her in the garden!" said Ron. "Anyway, she's not supposed to come into school anymore, Hagrid said Dumbledore banned her...."
Harry Potter and the Goblet of Fire, chapter 24: "Rita Skeeter's Scoop"
It did happen after the first task of the Tri Wizard tournament, and it was probably due to the way she was snooping around and harassing people.
For more elaborate information, please see Why didn't Rita Skeeter appear on the Marauder's Map?